Certain gas mixtures contain both carbon dioxide and hydrogen sulfide. For example, the overhead carbon dioxide stream produced in the distillative separation of carbon dioxide from ethane described in our copending application Ser. No. 131,416, filed Mar. 18, 1980, might contain hydrogen sulfide in addition to carbon dioxide for many sources of the gas stream processed. Other sources of gas mixtures which might contain high percentages of carbon dioxide together with hydrogen sulfide are: natural occurring gas and associated gas mixtures; coal gasification or liquefaction crude gas products; and sulfur processing "tail" gases.
Recently, there has been an increased interest for certain industrial applications to resolve mixtures of carbon dioxide and hydrogen sulfide into relatively pure fractions. Many times, in enhanced oil recovery situations, for example, it is desirable to employ carbon dioxide having less than about 100 ppm hydrogen sulfide. It is also desirable to remove hydrogen sulfide from carbon dioxide streams which are vented. Relatively high purity hydrogen sulfide streams are also desired or necessary, for the production of elemental sulfur, by processes such as the Claus sulfur process.
Previously, the separation of carbon dioxide and hydrogen sulfide into two product streams, one containing carbon dioxide without a significant amount of hydrogen sulfide and one containing virtually all of the hydrogen sulfide, was difficult and extremely costly by distillative techniques. This is due to the relatively close volatilities of carbon dioxide and hydrogen sulfide at high carbon dioxide concentrations. See Bierlein, J. A. and Kay, W. B., Industrial and Engineering Chemistry, Vol. 45, No. 3, "Phase-Equilibrium Properties of System Carbon Dioxide-Hydrogen Sulfide", pages 618-624 (1953). Bierlein and Kay determined the phase-equilibrium properties of carbon dioxide/hydrogen sulfide systems and concluded that no azeotrope existed. Nevertheless, there was evidence that intermolecular forces of the kind causing azeotrope formation were strongly developed at the carbon dioxide-rich end of the system, together with a very flat terminal slope, which suggested a strong tendency towards formation of a minimum-boiling mixture. Based upon these data, Bierlein and Kay concluded that separation of carbon dioxide from hydrogen sulfide in the binary system became very difficult above 0.8 mole fraction of carbon dioxide, and would require a large number of theoretical stages for further separation.
These predictions are born out and the difficulty of separating carbon dioxide from hydrogen sulfide by distillation is illustrated in U.S. Pat. No. 3,417,572 issued to Pryor. In the Pryor invention, a distillation column is employed to separate a mixture of hydrogen sulfide and carbon dioxide into a hydrogen sulfide bottoms product stream and a carbon dioxide overhead product stream. Although the overhead carbon dioxide stream obtained has high purity, the bottoms product stream of hydrogen sulfide has a hydrogen sulfide concentration minimally adequate as a feed to a Claus process, which may be as little as slightly over 10 mole percent hydrogen sulfide. Even to obtain this separation, it is indicated that 100 trays were used in the distillation column.
In view of the difficult nature of complete distillative separations of carbon dioxide from hydrogen sulfide, such separations have been accomplished commercially primarily by solvent extraction techniques. Solvent extraction techniques are costly, albeit less so than prior distillative techniques.